The modern world depends heavily on sea-going oil tankers for the delivery of oil and oil products in bulk from the area of; extraction to areas of refining or use. While sea-going vessels remain a relatively inexpensive method of transporting large quantities of such products, they are subject to special risks, particularly the risk of collision with another vessel or an obstruction which may result in spillage of oil products into open waters. While statistically such rupture to the vessel resulting in spillages are rare, the catastrophic effects of any such spillages have made this risk a very major world environmental problem. As a famous example, the EXXON VALDEZ accident in Prince William Sound, Alaska, in 1989 caused damage to the environment not fully repaired, even though billions of dollars have been spent in clean-up efforts.
Traditionally, a sea-going oil tanker has a single skin, fabricated from steel, protecting the cargo from the surrounding sea. After the EXXON VALDEZ and other similar accidents, there has been increased interest in designing oil-carrying ships less likely to cause environmental injury resulting from skin rupture, whether from collision with another object or because of excessive stresses put upon the vessel's structure by the sea. Suggestions have included a variety of designs and techniques, including double hulls, hydrostatic balancing of cargo and the like (for background, see Tanker Spills--Prevention by Design, National Research Council (US) Committee on Tank Vessel Design, 1991, National Academy Press, Washington, D.C.). But in particular, any cargo protection system relying only on a rigid, rupturable membrane such as steel is inherently at risk of failure in high-energy impacts resulting from grounding or from collision with rocks or other vessels, or may fail from metal fatigue or other age-related weaknesses.
It has been suggested that a flexible membrane be used to protect oil and similar cargoes from spillage in case of an accident. For example, U.S. Pat. No. 4,347,798 to Gallagher (1982) teaches the use of a closed buffer tank, of flexible material such as a nylon-based elastomer, positioned outboard of the cargo spaces and carrying salt water or other ballast liquids. Gallagher employs a single flexible bladder for each cargo-carrying compartment, segregated from and positioned outboard of the cargo space. The outer wall of that bladder is permanently sealed to the inner wall of the hull.
The protection provided by Gallagher is limited, since any impact rupturing the hull may well also rupture the flexible membrane sealed to it, thus leaving only a single membrane protecting the cargo.
My invention employs simple, readily-installable and removable modules each comprising a combination of a flexible cargo tank and segregated flexible tank made of a tough but resilient material. The bladder configuration is not sealed to the hull. Each bladder system provides within its design the equivalent of a double-side, double-bottom hull, formed by an outer ballast/buffer tank space which is adapted to be pressurized and also to carry a ballast liquid if desired. In case of an impact which ruptures the vessel's steel hull, the resilient walls of the bladder are adapted to deflect and yield in order better to absorb a high-energy impact and deter or eliminate destructive further rupturing, all as described below.
The bladder membrane system is fabricated of a composite of materials that have been researched in depth. Some of these membrane composites are a fraction of the weight of steel and of significantly greater impact resistance.